1. Field of the Invention
The present invention relates to glass melters. In particular, the present invention relates to electrodes for heating the space above the melt in a glass melter.
2. Discussion of Background
A glass melter is a refractory vessel into which glass frit is fed and heated by electrodes until it melts. The molten glass is called a melt. The top of the melt, called a cap, being on the periphery of the melt where the temperature is lower than the center of the melt and where the newly-introduced, unmelted frit joins the molten glass, forms a crust. The cap can insulate the newly added frit from the hot center of the melt and affects the temperature distribution of the melt and, hence, broadens the range of characteristics of the glass produced thereby. Primary electrodes are submerged in the molten glass. The glass is heated as the glass conducts current between the electrodes. Additional electrodes are sometimes also placed in the zone above the melt, the "vapor space" or "plenum", to heat the cap. These electrodes are called lid heaters.
Electrodes of various designs are used in connection with melters. Many have cores made of one material surrounded by sheaths of a second material. Nickel and nickel alloys, because of good conductivity and corrosion resistance, are frequently used. One nickel alloy sold under the trademark INCONEL 690, is described as useful for electrodes in U.S. Pat. No. 4,627,069 issued to Harvey, et al. They state that INCONEL 690 is a useful material for electrodes in melters because it is highly resistive to corrosion and heat, and highly conductive electrically.
Sometimes several electrodes are connected electrically, such as is described by Ohnsorg, et al., in U.S. Pat. No. 3,518,351. Ohnsorg, et al connect hollow electrodes electrically in parallel for use in a glass melter. Plugs are used in the ends of these tubular heaters to avoid the "chimney" affect. The material that connects the electrodes together is selected to have a lower resistivity than the electrode tube material.
For high quality glass and for efficient glass melting, however, it is necessary to heat the plenum with lid electrodes so that cap formation is minimized. There remains a need for efficient, long-lasting electrodes for this purpose.